Accomplishing an 80 miles per gallon fuel economy in an affordable vehicle by early in the next century will require developmental breakthroughs in a number of areas and a high probability of several significant inventions. Ideally, a vehicle with an 80 mpg fuel economy will be efficient, clean, affordable and will be competitive in deliverable performance with today's models such as a Chevrolet Lumina, Ford Taurus, or Chrysler Concorde.
Current automotive passenger car practice is to size the engine large enough to provide the desired acceleration performance at both low speed (standing start) and high speed (passing performance in level and hilly terrain), and then by the selection of gear ratios, to attempt to tailor this large engine to maximize its fuel economy. This practice generally causes the engine to operate most of the time at load points of 10-30% of maximum power, well off from the engine's maximum efficiency point.
In order to overcome this drive efficiency shortcoming, hybrid drive systems have been devised. A series hybrid system is one in which the total power output of the primary engine power source is directed through an intermediate holding device which combines that energy with energy from secondary sources such as that recovered from regenerative braking. The energy is then distributed as required to meet the demands of the vehicle and driver.
Most serious contenders for high fuel economy drive systems up to this time have expressed a preference for a series hybrid system with an undefined storage capability of at least 0.5 kW which acts as a buffer between the power source and the vehicle propulsion system. The advantages of such a system are that the primary power source can be precisely tailored to a specific single power level to maximize its efficiency, and regenerative braking compatible with a storage device can be easily factored into the energy equation. The disadvantages are that additional losses are introduced with multiple interfaces, as the energy finds its way from the power source to the storage device to the propulsion system through several energy converters and controllers.
The requirements of storing 0.5 kW of energy and also providing high braking and propulsion power on the order of 50-60 kW rule out a number of storage systems such as hydraulic accumulators and batteries as presently known. Accumulators are unsuitable because of their limited energy storage capability relative to their size, and batteries because of the high power discharge requirement. This leaves capacitors and flywheels, both of which require inventions to bring them to fruition. High efficiency power converters and controllers are required at both interfaces: engine to storage and storage to propulsion.
The mainstream of the Partnership For New Generation Vehicles (P.N.G.V.) government/industry cooperative programs for hybrid vehicles is directed toward series hybrid electric vehicles because it has not been heretofore demonstrated that any other (parallel) hybrid arrangement will accomplish the goal of fuel efficiency at three times the current industry average.
A parallel hybrid drive system is one which retains two parallel power paths for at least two power sources. Consideration of a parallel hybrid system which keeps a more conventional mechanical driveline in place opens the door for consideration of current technology hydraulic regenerative energy recovery with hydraulic accumulator storage devices having less than 0.1 kW capacity because of the lessened energy and power requirements. The need for an invention now shifts to the powerplant where a much broader high efficiency power spectrum is needed. As has been pointed out by Ross and Wu (SAE 950958), engine mechanical loss is the largest single source of wasted energy between the fuel tank and the road.